The present invention relates to a liquid crystal display, and more particularly to a circuit using thin film transistors (referred to as "TFT" hereinafter) for enabling a matrix type liquid crystal display.
In 1972, Westinghouse Electric Co. introduced a new concept of a matrix type liquid crystal display panel wherein a transistor and a parasitic capacitor were implemented with thin film technology for each of picture elements.
While details of specification of such panel are fully apparent from "A 6".times.6"20 1/inch Liquid Crystal Display Panel", by T. P. Brody et al, IEEE Trans. on Electron Devices EC-20 P 995, 1973, its structure and operating principle will be briefly discussed in according to such technical article to give a better understanding of the present invention.
FIG. 1 shows a circuit including a TFT and a parasitic capacitor for a respective one of picture elements of the panel and FIG. 2 shows enabling voltage waveforms associated with the circuit of FIG. 1. In the given example the picture elements are only four and aligned in an X-Y matrix to provide a visual matrix display through a proper wiring pattern.
If the source voltage V.sub.1 and the gate voltage V.sub.1 are applied from the source electrode 1 and the gate electrode 2 of FIG. 1, then the TFT 3 is placed into the conductive (ON) state so that the parasitic capacitor C.sub.S 5 in parallel with the capacitor C.sub.LC 4 of the liquid crystal material is charged via the ON resistance R.sub.ON of the TFT from the source electrode 1. Therefore, the potential (V drain 1) at the drain electrode 6 varies pursuant to the following formula (1): EQU V drain 1=V.sub.1 (1-e-t/.tau.1) (1)
wherein EQU .tau..sub.1 =RON (C.sub.LC +C.sub.S)
Then, if the gate voltage at the gate electrode 2 is charged to -V.sub.2, the TFT 3 is turned to the cut-off (OFF) state. This leads to that the capacitors C.sub.LC and C.sub.S 5 start discharging the cumulative charge thereon through the OFF resistance R.sub.OFF of the TFT 3 and the resistance R.sub.LC of the liquid crystal material. Since the resistances R.sub.OFF, R.sub.LC and R.sub.ON are correlated as follows, EQU R.sub.OFF &gt;&gt;R.sub.ON, R.sub.LC &gt;R.sub.OFF
the process of discharging goes on quite slowly such that the potential (V drain 2) at the drain electrode is held considerably high for a relatively long period of time as defined by the following formula: EQU V drain 2=V.sub.1 e-t/.tau.2 (2)
wherein EQU .tau..sub.2 =(R.sub.OFF //R.sub.LC) (C.sub.LC +C.sub.S)
As the history of these processes is apparent from the voltage waveform chart of FIG. 2, the effective voltage at the drain electrode, namely, the effective voltage developing across the liquid crystal unit element is remarkably high and assures a high contrast display irrespective of the voltage applied to the source electrode 1 with a small duty factor and a very low effective value.
The cell structure which operates pursuant to the above described principle is illustrated in FIG. 3, which generally comprises a thin film transistor array substrate 22 and a counter substrate 23. The former carries the TFT 3, the capacitor 5 and one electrode 11 deposited on a glass support 7 by a well known evaporation method and aligned in the X-Y coordinates with X and Y leads for each of the liquid crystal unit elements, whereas the latter carries an entire transparent, conductive film 17 common to all the unit elements deposited on another glass support 7'. Both electrode substrates are subject to a conventional TN (twisted nematic) alignment process such as slant evaporation and rubbing after transparent insulating layers 14 and 15 of SiO, SiO.sub.2, etc. are deposited thereon. In addition, both substrates are bonded together via a sealing member 21 and a proper liquid crystal material such as TN-FEM liquid crystal and guest host effect liquid crystal is injected therebetween, completing the fabrication of a matrix type liquid crystal display panel using the TFTs. At last, a pair of polarizers 18, 19' and a reflector 2 are disposed outside the matrix type liquid crystal panel.
In FIG. 3, 8 designates the gate electrode of Al or the like; 9 designates an electrode of the capacitor C.sub.s ; 10 designates a gate insulating film of the TFT and a dielectric film of the capacitor C.sub.s ; 11 designates an electrode pad of the liquid crystal element; 12 designates the source electrode; 13 designates the drain electrode; and 24 designates a semiconductor layer.
Nevertheless, two basic problems have been experienced in enabling the above illustrated display panel with enabling voltages as indicated in FIG. 2.
(1). When viewing waveforms of enabling voltages as indicated in FIG. 4 to enable all the picture elements other than one selected by a specific source electrode S.sub.i and a specific gate electrode G.sub.j, the TFT for the selected or disabled one remains in the cut-off (OFF) state but the capacitors L.sub.LC +C.sub.S are progressively charged via the OFF resistance R.sub.OFF. It is therefore possible that a voltage more than a given threshold voltage level V.sub.th of the liquid crystal material may be applied thereto. The resulting voltage is in the form as indicated in FIG. 4 and on-off switching is effected between the source voltage V.sub.si and the gate voltage V.sub.Gj so that, while the TFT is in the OFF state, the drain voltage V.sub.Dij bears an increased effective value equal to or higher than that in the ON state. This causes an objectionable visual display or a difference in contrast corresponding to the number of the liquid crystal elements enabled at this moment.
(2). As long as the voltage-current characteristics (V.sub.D -I.sub.D) of the TFT are symmetric with respect to the polarity, an ideal waveform of drain voltage which contains no d.c. component is available as shown in FIG. 2(C). However, the operating characteristics (V.sub.D -I.sub.D) of conventional TFTs are in fact asymmetic with respect to the polarity as viewed from FIG. 6. The waveform of the drain voltage, namely, the voltage applied across the liquid crystal material is further asymmetic as viewed from FIG. 2(d). Thus, there is the likelihood of supplying the liquid crystal material with a voltage inclusive of a d.c. component and shortening life of the liquid crystal panel.
Accordingly, in order to overcome these two problems, Westinghouse Electric Co. devoted the research activities to the development of TFTs with excellent operating properties in which R.sub.ON /R.sub.OFF .apprxeq.70,000 and thus leak current in the OFF state does not amount less than 5 nA whereas on current is as high as 350 .mu.A. Such attempt was not fully successful because of incomplete removal of the d.c. component.